This invention relates generally to a torque wrench adaptor and particularly to an adaptor for limiting torque applied to a mine roof bolt.
Roof bolting has proved an effective means of promoting safety in mine operations. It is generally believed that the effectiveness results from the fact that the bolts provide a means by which the various bedded planes are connected together to form a beam which prevents roof falls and that the sagging of the roof is prevented because the tension in each bolt tends to increase the friction between the layers thus minimizing horizontal pressures. Understandably it is of great importance that the correct torque be applied to the roof bolts, particularly in the case of conventional roof bolts which consist of an expansion shell, a tapered nut and a threaded bolt having a bearing plate mounted thereon. When such bolts are used the roof is predrilled to receive the shell and the bolt is generally torqued with a wrench operated by the drilling unit. If a roof bolt is not tightened sufficiently it cannot support the roof load. If it is over-tightened there is a possibility of damage to the material adjacent the expansion shell, or to the shell itself. The amount of torque applied by a hydraulic drill unit of the type commonly used is determined by a relief valve. Such valves are unreliable and, after the bolts are tightened, it is necessary to use a hand-operated torque wrench to test each bolt. Under-tightened bolts can be tightened, but those that have been over-tightened cannot be slackened off without the risk of damage to the roof.
Of course, torque wrenches in themselves are not new, and wrenches are known which utilize a clutch device to predetermine the point at which the wrench parts will slip. However, in general, the clutch drive connection of such wrenches is disposed between two parts of an interior drive shaft rather than between a drive shaft and an exterior housing, which results in a complicated structure. In addition, although torque wrenches are known which are capable of adjustment, the adjustment feature is invariably provided by relatively movable housing portions which are threadedly related to compress an internal spring against a clutch assembly. Such a structural arrangement of parts renders inadvertent adjustment likely.
The above disadvantages are avoided by the present device in a manner not disclosed in the known prior art.